fix: prevent nil deref on session reinstatement during cleanup race

When a new login arrives while the previous connection's async cleanup
is still running, LookupISCSISession finds the stale session but
LookupConnection returns nil (old connection already closed). This
caused a nil pointer dereference when accessing existConn.session
during session reinstatement.

Fix by checking existConn != nil before reinstatement. If the old
connection is already gone, unbind the stale session and create
a fresh one instead.

Also add sync.Once to removeConnectionFromSession to prevent
concurrent goroutine and main-path cleanup from racing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/port/iscsit/conn.go

@@ -88,7 +88,8 @@ type iscsiConnection struct {
 	rxTask *iscsiTask
 	txTask *iscsiTask
 
-	readLock *sync.RWMutex
+	readLock    *sync.RWMutex
+	cleanupOnce sync.Once
 }
 
 type taskState int